Finishing Knuckles's sprites? *shot*
Some enemy ai/boss ai that doesn't really belong to any sprites, and are just there for us to have fun with, would be nice. Perhapes a Metal Sonic Esque boss? (He flies arround the arena shooting energy balls at you, and desperation is him charging at you, invincible while doing it. He comes down to the ground after every few attack rounds to allow you to hit him. You could just use GFZ eggman sprites as placeholders or something if you wish)
Some extra, empty enemy, object, and boss slots would also be nice. Like about 20-40 of each or so.